The present invention relates to a plate lockup apparatus, arranged in a plate cylinder of a printing press, for fixing leading and trailing ends of a plate wound around the circumferential surface of the plate cylinder.
A gap having almost a rectangular section and a length almost equal to the overall length of a plate cylinder is formed in the circumferential surface of the plate cylinder for each of a variety of printing presses. A plate lockup apparatus consisting of a leading-side lockup device for gripping the leading end of a machine plate (to be referred to as a plate hereinafter) and a trailing-side lockup device for gripping the trailing end of the plate, gripped by the leading-side lockup device and then wound around the circumferential surface of the plate cylinder, is fixed on the bottom surface of the gap to extend in the axial direction of the plate cylinder.
Each of the conventional leading- and trailing-side lockup devices comprises an elongated lockup table extending in the axial direction of the plate cylinder, and gripper plates, pivotally supported by this lockup table, for gripping or releasing the plate with or from the lockup table by being opened or closed as they pivot. In the vicinity of notches formed in the gripper plates, a cam shaft having a plurality of cams which can be respectively engaged with these notches extend along the axial direction of the plate cylinder. If the apparatus is of the spring closed type, compression coil springs are interposed between the gripper plates and the lockup table to bias the gripper plates toward the lockup table in a closing direction. In addition, a plate tightening unit having compression coil springs for biasing the gripper plates in a plate tightening direction is provided to the trailing-side lockup device.
With the above arrangement, in order to grip the plate, when the leading-side cam shaft is pivoted manually or by a drive unit, the gripper plates are urged by the cam surfaces of the cams and opened against the elastic forces of the compression coil springs. An end portion of the plate is inserted between the gripper surfaces of the gripper plates and the lockup table. When the cam shaft is pivoted in the direction opposite to the direction described above, the gripper plates are released from the opening operation by the cam surfaces and closed by the elastic forces of the compression coil springs, thereby gripping the leading end of the plate. Then, when the plate cylinder is rotated by almost one revolution, the plate is wound around the circumferential surface of the plate cylinder. Thus, the trailing-side lockup device is caused to grip the trailing end of the plate in a manner similar to that described above, and the trailing-side lockup device is moved along the circumferential direction of the plate cylinder by the plate tightening unit, so that the plate is brought into tight contact with the circumferential surface of the plate cylinder.
In the conventional plate lockup apparatus described above, the plate gripping force must be adjusted in accordance with the thickness or the like of the plate. In the conventional spring closed type plate lockup apparatus described above, to adjust the gripping force of the leading-side plate lockup device, the spring forces of the compression coil springs (described above) for biasing the gripping plates in the closing direction are adjusted. With this arrangement, however, since a multiple of compression coil springs must be adjusted one by one, a long period of time and much labor are needed. Also, the gripping force cannot be sufficiently changed by changing only the spring forces.
Furthermore, in a cam closed type plate lockup apparatus in which the gripping plates are pivoted in the closing direction by cams and in the open direction by the spring forces of spring members, to adjust the plate gripping force of the lockup device, shims are inserted between the cam surfaces of the cams and the gripper plates. Hence, a long period of time and much labor are needed, as in the above case.